We have been trying to reproduce this and so far no luck. We are testing this on our MacBook Pro and putting it to sleep and waking it up hours or days later. Hopefully at some point it will cause this issue that you guys are having and we can see if we can fix it.
Please keep a few things in mind, as was mentioned by a user. If you are setting an Alert and the Alert time is before the current date/time, you will not get an alert. So for example if you set an event for tomorrow, but your Alert is set to remind you two days before, you will not get that alert, because it is in the past. SM will only trigger on alerts that are from Now and forward. Also if you try to set an alert of 1 minute in advance, there is a chance that the Mac OSX will not send the event to SM fast enough and you will not get that alert.